Method for the preparation of fibrous soluble calcium sulfate anhydrite

ABSTRACT

A reinforced plastic article is described, in which the plastic matrix is an organic polymeric resin and the reinforcing fiber is fibrous soluble calcium sulfate anhydrite, the fibers thereof having specified dimensions. The fibers are produced by a process in which a dilute aqueous suspension of gypsum is first formed and converted to the fibrous soluble anhydrite by reaction in the presence of saturated steam at 140*C to 200*C for 0.5 to 3.0 hours.

United States Patent J aunarajs et al.

METHOD FOR THE PREPARATION OF FIBROUS SOLUBLE CALCIUM SULFATE ANHYDRITE Inventors: .Karlis L. Jaunarajs, Denver, Colo.;

Julie C. Yang, Lexington, Mass.

Assignee: Johns-Manville Corporation,

Denver, Colo.

Filed: Aug. 5, 1974 Appl. N0.: 494,426

Related US. Application Data Continuation-in-part of Ser. No. 462,911, April 22, 1974, abandoned, which is a division of Ser. No. 281,268, Aug. 16, 1972, Pat. No. 3,835,219.

US. Cl 260/40 R; 106/306; 260/37 R; 260/42.17

Int. Cl)- C08K 7/08 Field of Search 260/40 R, 42.17, 37 R; 106/306 References Cited UNITED STATES PATENTS 8/1943 McCord 106/306 Oct. 28, 1975 2,745,813 5/1956 Logemann 260/42.43 3,553,302 l/l97l Susuki ct all 260/42.46 3,711,438 1/1973 Susuki et al. 260/42.46

Primary Examiner-James H. Derrington Attorney, Agent, or FirmRobert M. Krone; James W. McClain [57] ABSTRACT 7 Claims, No Drawings METHOD FOR THE PREPARATION OF FIBROUS SOLUBLE CALCIUM SULFATE ANHYDRITE CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 462,911, filed Apr. 22, 1974, now abancloned which is in turn a division of application Ser. No. 281,268, filed Aug. 16, 1972 now US. Pat. No. 3,835,219.

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention herein relates to the formation of. fibrous soluble calcium sulfate anhydrite and to plastic articles reinforced therewith.

Research has been carried on for years on the conversion of gypsum (CaSO -2H O) into the various forms of its hemihydrate (CaSO /2H O), sometimes referred to as plaster of Paris, and its anhydrite (CaSO The anhydrite exists in two forms, commonly referred to as the soluble form and the insoluble form, on the basis of their relative solubilities in water at approximately ambient temperature. The soluble anhydrite can occur in more than one crystalline form, one of which is a fibrous form.

2. Description of Prior Art There is a wide variety of patents and articles discussing various aspects of soluble anhydrite formation from gypsum. Typical descriptive articles which describe the soluble anhydrite and its properties are those by Riddell in the May 1950 issue of Rock Products, page 68, and by Bauer in the April 1952 issue of Pit and Quarry, page 1 13. The book The Chemistry of Cement and Con- Crete by Lea describes properties of gypsum and its derivatives on page 18. Various aspects of the formation of soluble anhydrite by calcination of gypsum at high temperature in the presence of steam under pressure are disclosed in US. Pat. Nos. 1,931,240; 1,941,188; 1,989,712; 2,448,218; 2,460,267; and 3,579,300. Formation of fibrous hemihydrate from high concentration suspensions of gypsum is described in an article by Eberl et al. in the May 1949 issue of Industrial and Engineering Chemistry, page 1061. Crystal habit modifiers are described in US. Pat. Nos. 2,907,668 and 3,520,708.

BRIEF SUMMARY OF THE INVENTION The invention herein is a reinforced composite article comprising an organic polymeric resin matrix reinforced with fibrous calcium sulfate anhydrite. The fibers are produced by a novel and specific process for the formation of fibrous soluble calcium sulfate anhydrite, yielding fibers of specified dimensions. These fibers have been found to be quite useful for reinforcement of the organic polymeric matrices, for use of these fibrous soluble anhydrites as reinforcing agents substantially increases the strength of polymeric materials. Often the strength of the fibrous soluble anhydrite reinforced materials is significantly greater than that of similar polymeric materials reinforced by conventional fibrous materials such as asbestos. The process comprises first forming an aqueous suspension of gypsum which is not greater than 0.30 molar in gypsum. The gypsum used is ground or otherwise comminuted to a particulate form in which at least 90% of the particles are minus 4 mesh in size. The suspension may also contain a crystal habit modifier which is suitable for formation of the fibrous soluble anhydrite. Following formation of the suspension, the gypsum in the suspension is converted to the fibrous soluble anhydrite by reaction in a pressure vessel in the presence of saturated steam at a temperature in the range of from 140C to 200C for a period of 0.5 to 3.0 hours, thereby forming fibers of soluble calcium sulfate anhydrite having an overall average length in the range of from 25 to microns, individual maximum lengths generally not exceeding 150 microns, and aspect ratios in the range of from 10:1 to :1.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS The invention herein is a reinforced composite article comprising an organic polymeric resin matrix reinforced with fibrous calcium sulfate anhydrite. The fibers are produced by a process in which fibrous soluble anhydrite may be produced to the virtual exclusion of nonfibrous soluble anhydrite, insoluble anhydrite, and- /or calcium sulfate hemihydrate. The fibers produced by the process of this invention serve as excellent reinforcing agents for the organic polymeric matrices, significantly increasing the strength thereof.

The invention herein contemplates the incorporation of the fibrous soluble anhydrite into organic polymeric resin matrices. A wide variety of organic resins are suitable as matrices which can be reinforced by the fibrous soluble anhydrite. These include polyesters, polyolefins such as polyethylene and polypropylene, poly(viny1 halides) such as poly(vinyl chloride), and polystyrene. The weight ratio of polymeric resin to fibrous soluble anhydrite will be in the range of from about 2:1 to about 12:1, preferably 2:1 to 6:1. If desired, the polymeric matrix can be extended by addition of up to about 50% of filler. Also included may be small amounts of conventional stabilizers, antioxidants, pigments, etc., used in such polymeric matrices.

The process for the formation of fibrous soluble calcium sulfate anhydrite, the fibers thereof having an overall average length in the range of from 25 to 70 microns, individual maximum lengths not exceeding 150 microns, and aspect ratios in the range of from 10:1 to 100:1, comprises: forming an aqueous suspension of gypsum, said suspension being not greater than 0.30M in gypsum, and said gypsum prior to dispersion being in the form of particles at least 90% of which are minus 4 mesh in size; and converting the gypsum in said suspension to said fibrous soluble anhydrite by reaction in a pressure vessel in the presence of saturated steam at a temperature in the range of from C to 200C for a period of 0.5 to 3.0 hours, thereby forming fibers of soluble calcium sulfate hemihydrate having an overall average length in the range of from 25 to 70 microns, individual maximum lengths generally not exceeding microns, and aspect ratios in the range of from 10:1 to 100:1.

In the process herein, a specified form of fibrous soluble calcium sulfate anhydrite is formed from gypsum. The process is carried out in a conventional pressure vessel (such as an autoclave) which is preferably equipped with means for stirring the aqueous suspension of gypsum. The pressure vessel also is equipped with means for supplying saturated steam to the autoclave in order to maintain the proper temperature and pressure within the vessel.

In the pressure vessel, the temperature during the reaction period is maintained in the range of 140C to 200C, preferably in the range of 140C to 160C. Since this temperature represents the temperature of saturated steam, there will be associated the pressure of the saturated steam at the given temperature. Consequently, the attendant steam pressure will be approximately 52 to 225 psia, preferably about 52 to 90 psia.

The basic raw material used is gypsum, calcium sulfate dihydrate. The gypsum used herein may be pure or substantially pure gypsum or it may be a commercial grade containing a small amount of impurities (such as calcite) which do not significantly affect the formation of the fibrous soluble anhydrite. Best results are obtained when the gypsum is ground, crushed, or otherwise comminuted to a particulate form in which at least 90% of the particles are of minus 4 mesh in size. Preferably approximately 80% of the particles will be minus 60 mesh in size. The comminuted gypsum is then suspended in water to form an aqueous slurry which has a concentration not greater than 0.30M in gypsum and preferably not greater than 0.26M. At greater cncentrations, the slurry has been found to be too thick to produce a uniform degree of formation of the fibrous soluble anhydrite. For instance, suspensions of 0.35M and 1.17M in gypsum have been found to yield reaction products containing significant amounts of nonfibrous hemihydrate in the first step of the present process; this results in an unsatisfactory yield of fibrous soluble anhydrite in the subsequent step of the process.

The suspension may also contain a small amount of a crystal habit modifier which is suitable for the formation of the fibrous soluble anhydrite. These are generally acids such as boric, succinic, adipic, malonic, sebacic, and similar acids or their salts, as well as salts such as sodium chloride, sodium sulfate, aluminum sulfate, and zinc sulfate. Preferred among these is boric acid. The crystal habit modifier will be present in an amount of from 0.1 to 5 weight percent, preferably 0.25 to 1.5 percent.

As noted above, the pressure vessel in which the reaction is carried out should be equipped with means for stirring or otherwise mildly agitating the gypsum slurry during the reaction. Such stirring may be continuous or intermittent throughout the reaction period.

Reaction time will be in the range of 0.5 to 3.0 hours, preferably 0.75 to 2.5 hours. Reaction times will be inversely related to the temperature in most cases, with longer reaction times necessary for complete conversion to the fibrous soluble anhydrite at the lower reaction temperatures. Since conversion to the fibrous soluble anhydrite is virtually complete at temperatures within the above range in 3 hours, and usually within 2.5 hours, heating for a longer period is normally not justified.

The following examples are illustrative of the formation of fibers.

EXAMPLE 1 A commercial gypsum containing a minimum calcium sulfate content of 64.5% and being in particulate form in which 98% of the particles were of minus mesh in size, was suspended in water to form a 0.23M suspension. The suspension also contained 1.0 weight percent of boric acid. On reaction for 2 hours in the presence of saturated steam at an average temperature of about 145C a slurry was formed containing essentially all fibrous soluble anhydrite. The fibrous soluble anhydrite had individual fiber lengths in the range of from 10 to 60 microns with an average fiber length of 30 microns. The reaction was conducted in a commercial 1 gallon capacity stainless steel autoclave equipped with a stirrer. The stirrer was run at 300 rpm during the entire 2 hour reaction period.

EXAMPLE 2 The reaction conditions of Example 1 were repeated with the exception that the reaction time was extended to 140 minutes. The product, which was essentially all fibrous soluble anhydrite, had individual fiber lengths in the range of from 10 to microns with an average fiber length of 30 microns.

EXAMPLE 3 The commercial gypsum of Example 1 was suspended in water to form a 0.23M suspension. No boric acid was added. After reaction for 60 minutes at 150C a product composed essentially all of fibrous soluble anhydrite was formed. The individual fiber lengths were in the range of 20 to 150 microns, with an average length of 60 microns.

EXAMPLE 4 The commercial gypsum of Example 1 was suspended in water to form a 0.20M suspension. No boric acid was added. After reaction for 30 minutes at an average temperature of about 152.5C a product was formed which was composed essentially all of fibrous soluble anhydrite. The individual fiber lengths were in the range of 20 to 140 microns with an average fiber length of 60 microns.

What we claim is:

l. A reinforced composite article comprising an organic polymeric resin matrix reinforced with fibrous calcium sulfate anhydrite produced by a process which comprises:

a. forming an aqueous suspension of gypsum, said suspension being not greater than 0.30 molar in gypsum, and said gypsum prior to dispersion being in the form of particles at least of which are minus 4 mesh in size; and

b. converting the gypsum in said suspension to said fibrous soluble anhydrite by reaction in a pressure vessel in the presence of saturated steam at a temperature in'the range of from C to 200C for a period of 0.5 to 3.0 hours, thereby forming fibers of soluble calcium sulfate anhydrite having an overall average length in the range of from 25 to 70 microns, individual maximum lengths not exceeding microns, and aspect ratios in the range of from 10:1 to 100:1.

2. The reinforced composite article of claim 1 wherein said organic polymeric resin matrix comprises a resin selected from the group consisting of polyesters, polyolefins, poly(vinyl halides) and polystyrene.

3. The reinforced composite article of claim 2 wherein said resin is a polyolefin selected from the group consisting of polyethylene and polypropylene.

4. The reinforced composite article of claim 2 wherein said poly(vinyl halide) is poly(vinyl chloride).

5. The reinforced composite article of claim 1 wherein the weight ratio of polymeric resin to fibrous soluble anhydrite is in the range of from about 2:1 to about 12:1. 7

6. The reinforced composite article of claim 5 wherein said weight ratio is in the range of 2:1 to 6:1.

7. The reinforced composite article of claim 1 wherein said polymeric resin is extended by addition of up to about 50% of filler. 

1. A RESINFORCED COMPOSITE ARTICLE COMPRISING AN ORGANIC POLYMERIC RESIN MATRIX REINFORCED WITH FIBROUS CALCIUM SULFATE ANHYDRITE PRODUCED BY A PROCESS WITH COMPRISES: A. FORMING AN AQUEOUS SUSPENSION OF GYPSUM, SAID SUSPENSION BEING NOT GREATER THAN 0.30 MOLAR IN GYPSUM, AND SAID GYPSUM PRIOR TO DISPERSION BEING IN THE FORM OF PARTICLES AT LEAST 90% OF WHICH ARE MINUS 4 MESH IN SIZE, AND B. CONVERTING THE GYPSUM IN SAID SUSPENSION TO SAID FIBROUS SOLUBLE ANHYDRITE BY REACTION IN A PRESSURE VESSEL IN THE PRESENCE OF SATURATED STEAM AT A TEMPERATURE IN THE RANGE OF FROM 140*C TO 200*C FOR A PERIOD OF 0.5 TO 3.0 HOURS, THEREBY FORMING FIBERS OF SOLUBLE CALCIUM SULFATE ANHYDRITE HAVING AN OVERALL AVERAGE LENGTH IN THE RANGE OF FROM 25 TO 70 MICRONS, INDIVIDUAL MAXIMUM LENGTHS NOT EXCEEDING 150 MICRONS, AND ASPECT RATIOS IN THE RANGE OF FROM 10:1 TO 100:1.
 2. The reinforced composite article of claim 1 wherein said organic polymeric resin matrix comprises a resin selected from the group consisting of polyesters, polyolefins, poly(vinyl halides) and polystyrene.
 3. The reinforced composite article of claim 2 wherein said resin is a polyolefin selected from the group consisting of polyethylene and polypropylene.
 4. The reinforced composite article of claim 2 wherein said poly(vinyl halide) is poly(vinyl chloride).
 5. The reinforced composite article of claim 1 wherein the weight ratio of polymeric resin to fibrous soluble anhydrite is in the range of from about 2:1 to about 12:1.
 6. The reinforced composite article of claim 5 wherein said weight ratio is in the range of 2:1 to 6:1.
 7. The reinforced composite article of claim 1 wherein said polymeric resin is extended by addition of up to about 50% of filler. 